What is the sign 'to repeat the bar line'?

The sign 'to repeat the bar line' is typically represented by a double bar line, which indicates the end of a section or piece of music. It often suggests that the preceding material should be played again. This sign can be particularly useful in sheet music to streamline the performance of repeating sections without having to rewrite the notes. In some contexts, it may also signal a change in key or time signature following the repeat.